1. Field of the invention
The field of the invention is that of remote sensing instruments using optical aperture synthesis and in particular those embarked on satellites.
2. Description of the prior art
The person skilled in the art knows that remote sensing of extended scenes (or sources), such as the Earth or a portion thereof, for example, at high resolution from space, and in particular from high orbits, necessitates telescopes of large diameter, typically greater than ten meters for a resolution of one meter from geosynchronous orbit. The difficulty of producing mirrors and their weight increase with their diameter. Also, because the capacity of launch vehicles (determined in particular by the volume of their nose-cap) naturally limits the size of the onboard instrumentation, it is difficult to envisage the construction of large remote sensing instruments using monolithic mirrors.
To solve the above problems, it has been proposed to produce remote sensing instruments using optical aperture synthesis, which consist of at least two light collecting means, such as “small” mirrors, for example, which are generally plane, or portions of a “large” mirror, or telescopes, for directing light beams that they collect coming from the observed scene to an interferometer type beam combination optic delivering a remote sensing beam to a detector.
The benefit of the above type of instrument, over and above the fact that it can be embarked, lies in the fact that its detector can deliver images whose resolution is better (and which therefore offers more detail) than could be achieved by each light collection means on its own and compared to that which a monolithic mirror of large diameter could produce.
The collection means, which form the pupil of the remote sensing instrument, must of course be disposed with a particular geometry defined by frequency plan energetic filling rules. This therefore imposes compliance with constraints such as an isotropic (or quasi-isotropic) optical transfer function (OTF), absence of areas of cancellation in the OTF, and minimum redundancy of the observed frequencies.
However, when the instrument is embarked, to perform a selected remote sensing mission, other important parameters must also be taken into account.
A parameter such as the field of view of the instrument is strongly dependent on the types of light collecting means and recombination means employed. The field of view may in particular prove to be smaller than that offered by a conventional instrument (including a large monolithic mirror). This is the case in particular if the light collecting means and the recombination means define a Michelson interferometer, because of aberrations introduced by the many optical components and by virtue of the recombination principle.
The overall size is another important parameter to be taken into account. Using light collecting means and recombination means defining a Fizeau interferometer produces a field of view comparable to that of a conventional instrument but leads to a large overall size.
The number of light collecting means used is another important parameter. The probability of failure is a function of the complexity of the remote sensing instrument and therefore of the number of light collecting means.
No prior art remote sensing instrument proving entirely satisfactory, an object of the invention is therefore to improve on this situation.